In general, functions which record events relating to cardiac activity have come to be known as Holter functions. Holter functions in an implantable device, which a user can interrogate at some later date via telemetry with an external programming and display device, have come to be known as implantable Holter functions. Telemetry refers to the wireless communications between the external programmer and the implanted medical device, which techniques are well known.
Cardiac signals measured with external electrodes are called electrocardiograms, whereas heart signals measured with implanted electrodes are called electrograms.
As used herein, a defibrillator refers to any device intended to revert a tachyarrhythrnia with electrical energy substantially exceeding the energy provided by implantable cardiac pacemakers. This includes implantable defibrillator/cardioverter/pacemakers and implantable defibrillator / pacemakers. It should be understood that the present invention is applicable to such defibrillators as well as pacemakers which also perform a Holter function and devices that only perform a Holter function.
Holter functions for implantable medical devices have appeared in various forms in the prior art. U.S. Pat. No. 4,295,474 to Fischell, filed on Oct. 2, 1979, refers to an electrogram recording system, digitally storing in a first location the electrogram covering a period just prior to and including the occurrence of some physiological event, and in a second location storing the electrogram for a second period following the event. Fischell continuously stored the electrogram and "froze" it when the trigger event occurred, or at some predetermined time after the trigger event.
At the VIth World Symposium on Cardiac Pacing in Montreal, October 2 to 5, 1979, Attuel, Mugica, et al. presented an implantable pacemaker with a Holter function, which counted pauses in the heart rhythm. They proposed a future system which would identify bradycardia or tachycardia and record atrial and ventricular signals when detecting these arrhythmias.
In a publication entitled "Technologie des memoires et function Holter implantable", presented at Cardiostim in October 1980, Ripan and Jacobson discussed partitioning an available Holter memory in an implantable pacemaker. The pacemaker included a detector to classify arrhythmias. They proposed three schemes for storing information: (1) A two-second electrogram, triggered by a selected type of detected arrhythrnia; (2) Sets of occurrence counters for selected types of arrhythmia, each set corresponding to a period of time in the history of the device; and (3) Histograms of cardiac intervals, each histogram also corresponding to a period of time.
U.S. Pat. No. 4,374,382 to Markowitz, filed in January 1981, refers to a system called marker channel telemetry. Markers consist of digital codes corresponding to cardiac events, such as atrial sensing, ventricular pacing, etc. The Markowitz device transmitted these codes via telemetry to an external receiver, as the corresponding events occurred. Markowitz did not durably store the marker codes in the implant for display when the user desires.
U.S. Pat. No. 4,513,743 to van Arragon, Mensink, et al., filed in November 1982, refers to an implantable pacemaker with cardiac interval histograms each corresponding to a period of time, similar to the concept first presented by Ripart and Jacobson in 1980 (supra).
U.S. Pat. No. 5,052,399 to Olive and Lincoln, filed in September 1990, refers to a method for encoding and compressing a record of consecutive cardiac cycle lengths, wherein the device stores the difference between lengths of successive cycles, or if this exceeds the capacity of a memory word, it stores the last cycle length.
ELA Medical, the assignee of this application, has since November 1991 sold a pacemaker under the tradename Chorus II, which includes an implantable Holter function called marker chains. The device stores a marker and then an item of information permitting one to determine the time when the marker occurred relative to the previous marked event. It stores a chain of such time-stamped markers, permitting an external programmer device to reconstruct a synthesized timing diagram of marked events, with the intervals between them.